1. Field of the Invention
The present invention relates to an automatic focus adjustment technique suitable for use in an imaging apparatus such as a video camera having an interchangeable lens unit.
2. Description of the Related Art
In recent years, AF (auto focus) devices for video camera, that determine an AF evaluation value by detecting the sharpness of an image from an imaging signal and shift a focus lens to a position where the AF evaluation value becomes the highest to thereby perform focus adjustment, have been prevailing. Hereinafter, the above method is referred to as a “TVAF method”. As a TVAF evaluation value, the high frequency component level of a visible-image signal extracted by a band-pass filter for a predetermined frequency band is typically used. When an object is captured by the imaging system, a TVAF evaluation value increases as the focus lens is being focused and reaches a maximum at a focal point as shown in FIG. 2A. In other words, FIG. 2A shows that the degree of focusing becomes lower with distance from the focal point. FIG. 2B shows an operation (hereinafter referred to as “reciprocating operation”) for determining a focusing direction based on the change in the TVAF evaluation value obtained when the focus lens is vibrated at micro intervals. In the reciprocating operation, the influence of the movement of the lens on a capturing screen is not noticeable, and thus, the reciprocating operation is particularly used for a camera for capturing moving images. “Reciprocating operation” is also referred to as “wobbling operation”.
On the other hand, in the case of interchangeable lens systems, the change in image magnification is great in some lens units for use upon performing a reciprocating operation, whereby the influence of the movement of a focus lens on a capturing screen may be noticeable. Japanese Patent Laid-Open No. 2010-271697 discloses an apparatus for use in an automatic focus adjustment type video camera having an interchangeable lens unit, where the apparatus passes information about the change in image magnification from the lens unit to a camera unit to thereby restrict the vibration amplitude during the reciprocating operation.
In the AF operation performed by the apparatus disclosed in Japanese Patent Laid-Open No. 2010-271697, the vibration amplitude is restricted in response to the change in image magnification, and thus, the difference between TVAF evaluation values obtained by the reciprocating operation decreases. Consequently, when the S/N (signal-to-noise) ratio for generating a TVAF evaluation value decreases, the direction detection accuracy is degraded.
Also, in auto focus (AF) control performed in lens-interchangeable type imaging apparatuses, an imaging apparatus (camera body) generates a focus signal indicating the contrast state of a visible image (i.e., the focus state of an imaging optical system) as disclosed in Japanese Patent No. 3943609. It is common to cause an interchangeable lens (lens unit) to shift a focus lens based on a focus signal acquired from a camera body to thereby perform AF control.
In AF control using the focus signal, an increase or decrease in focus signal is detected by the reciprocation driving (hereinafter referred to as “micro vibration”) of a focus lens by a slight amount in the close direction and the infinity direction so as to determine a focusing direction which is the direction of movement of the focus lens in which the focus signal increases. Then, the center position (hereinafter referred to as “vibration center”) of the micro vibration of a focus lens is shifted in the focusing direction. When the shift of the vibration center to the focusing direction is repeated for a predetermined number of times, it is determined that the direction is the true focusing direction and the focus position at which the focus signal is maximized is searched for by shifting the focus lens in the focusing direction at a constant speed. This operation will be referred to as “peak-climbing driving” during AF control. “Peak-climbing driving” is also referred to as “mountain-climbing driving”. The micro vibration of the focus lens is performed again near the focused position to thereby determine whether or not the focus lens is in the focused state.
In such a case, since a camera body generates a focus signal tailored to characteristics of a lens unit mounted thereto and AF control is performed by the lens unit, the accuracy of focusing by AF control is determined by the combination of the camera body and the lens unit.
However, in recent years, the number of pixels of an imaging element such as a CMOS sensor or the like provided on a camera body side has been dramatically increasing. Since the depth of focus becomes shallow in an imaging element having a fine pixel pitch, the accuracy of focusing needs to be improved in accordance with the recent trend.
However, as an actuator for shifting a focus lens, a stepping motor or the like that has a restriction on drive resolution is often typically used. Thus, the position control resolution of a focus lens is also limited. In this case, when the micro vibration of the focus lens (also including the shift of the vibration center) is performed near the focused position over a shallow depth of focus of an imaging element having a fine pixel pitch, a focusing fluctuation in which an image captured by the imaging element is close to or away from the focused state occurs.
FIG. 27 illustrates the movement of a focus lens during peak-climbing driving, where the focus lens position (focused position) is plotted on the horizontal axis and the focus signal value is plotted on the vertical axis. The focus signal forms a bell shape such that the value increases as it approaches the focused position. During peak-climbing driving, a focus lens is shifted toward the focused position, at which a focus signal value is maximized (peak), at constant speed so as to search the focused position.
For the shift of the focus lens shown in A of FIG. 27, a focus signal decreases beyond its peak, and thus, it is determined that the focus lens has already passed over its focused position to thereby end peak-climbing driving. Then, the focus lens returns to a position at which the focus signal reaches its peak to thereby start micro vibration. On the other hand, for the shift of the focus lens shown in B of FIG. 27, a focus signal decreases without reaching its peak, and thus, it is determined that an improper direction has been set as the focusing direction and the shift direction of the focus lens is reversed so that peak-climbing driving is continued. In the micro vibration after detection of the focused position at which the focus signal value reaches its peak by means of peak-climbing driving, the focus determination of whether or not the focus lens position falls within the range of the depth of focus indicated by a hatched region shown in FIG. 27 is performed. When the focus lens is in focus, the focus lens is stopped and AF control ends.
FIG. 28 illustrates how the micro vibration of a focus lens is performed near its focused position for focus determination, where time is plotted on the horizontal axis and focus lens position (focused position) is plotted on the vertical axis. Thin horizontal dotted lines denote a position (position corresponding to the control resolution of an actuator) at which a focus lens can be stopped and a hatched region indicates a range of the depth of focus. The depth of focus has a width which is equally spaced from the vibration center (here, the focused position) indicated by chain-double dashed lines in the close direction and the infinity direction. The vibration amplitude amount and the shift amount of the vibration center upon micro vibration are set smaller than the range of the depth of focus.
FIG. 28 shows how the vibration center of a shift amount corresponding to one drive step of a stepping motor is repeatedly shifted while a focus lens is being shifted from the vibration center toward the close side and the infinity side by a vibration amplitude amount corresponding to one drive step of the stepping motor. If the vibration center falls within the range of the depth of focus successively for a predetermined number of times (e.g., three times), the focus determination is made and the shift of the focus lens is stopped.
Here, the shift amount of the focus lens when the vibration center shifts is set to an amount (summation shift amount) in which the vibration amplitude amount is added to the shift amount of the vibration center. Thus, as shown near the middle of FIG. 28, the position of the focus lens after being shifted including the shift of the vibration center undesirably exceeds the range of the depth of focus, whereby a focusing fluctuation of an image may be recognized by a user.
If the shift amount of the focus lens including the shift of the vibration center can be set to be small by setting the vibration amplitude and the center shift amount to be extremely small, the focus lens can be sufficiently subjected to micro vibration within the width of the shallow depth of focus, whereby a focusing fluctuation of an image is not recognized by a user.
However, the minimum value of each of the vibration amplitude amount and the shift amount of the vibration center is determined by the position control resolution of a focus lens corresponding to the drive resolution of a stepping motor, and thus, cannot be set too small. Even if the vibration amplitude and the shift amount of the vibration center can be set small, the shift of the focus lens in the focusing direction becomes slow in AF control, resulting in a reduction in responsiveness to AF control.